It is proposed to investigate the endocrine control of calcium homeostasis in mammals (rat) and birds (Japanese quail) along the following lines: (1) the determination of calcitonin and parathyroid hormone levels by bioassay under various physiological, nutritional, surgical, and pharmacological conditions; (2) an investigation of the nature of the rapid hypercalcemic response to parathyroid hormone; (3) a study of the role of parathyroid hormone in the adaptive response of the gut to calcium deprivation; and (4) a study of the role of calcitonin in the hibernating ground squirrel (Citellus tridecemlineatus). Particular attention will be given to the relationship between plasma calcitonin levels and gonadal hormone status.